One type of a planar type heterojunction avalanche photodiode is disclosed in the U.S. Pat. No. 4,651,187. In the planar type heterojunction avalanche photodiode, an n-InP buffer layer, an n.sup.- -In.sub.0.53 Ga.sub.0.47 As light absorbing layer, an n-InGaAsP intermediate band-gap layer, an n-InP avalanche multiplication layer, and an n.sup.- -InP- cap layer are successively provided on an n-InP substrate. Further, a p.sup.+ -InP region is preferentially provided in the avalanche multiplication layer and the cap layer, and the p.sup.+ -InP region is surrounded at its outer periphery by a p.sup.- -InP guard ring region. A p-ring shaped electrode and an anti reflection film are provided on the top surface of the cap layer and the p.sup.+ -InP region, and an n- electrode is provided on the back surface of the substrate.
In operation, a reverse bias voltage is applied across the p-and n-electrodes, and light is supplied to the top surface of the anti-reflection film. Thus, electron and hole carriers are excited in the light absorbing layer, so that hole carriers are injected into the avalanche multiplication layer to result in carrier multiplication.
In the avalanche photodiode of a top surface illumination type as described above, a wide bandwidth characteristic is obtained in a case where the light absorbing layer, the intermediate band-gap layer, and the avalanche multiplication layer become thin in total thickness to shorten a carrier transit time, during which carriers produced in the light excitation and the light multiplication transit through the three layers. In this respect, the light absorbing layer and the avalanche multiplication layer are desired to be thinner because the intermediate band-gap layer is originally thin, that is, a carrier transit time is negligible in the intermediate band-gap layer. The wide bandwidth characteristic is further obtained in a case where an area of the p.sup.+ -InP region and the p.sup.- -InP guard ring becomes small to lower a junction capacitance.
In the planar type heterojunction avalanche photodiode, however, there is a disadvantage that the sensitivity is deteriorated because the quantum efficiency is lowered, when the light absorbing layer is made thin to shorten the aforementioned carrier transit time. There is a further disadvantage that a predetermined reduction of a junction capacitance is difficult even if an area of the p-ring shaped electrode becomes small, because a lead wire electrode pad is provided to connect a lead wire electrically to the p-ring shaped electrode. The reduction of the junction area is also difficult, because the light supplying surface and the lead wire electrode pad are positioned on the common top surface of the avalanche photodiode.